Prior known electrically rewritable semiconductor memory devices are generally categorized into volatile memories and nonvolatile memories. Whereas volatile memories include DRAMs and SRAMs, nonvolatile memories include EEPROM flash memories such as those of the NAND or NOR type or the like. The DRAMs and SRAMs are featured by high-speed random accessibility; the flash memories feature large capacity and long-term data retainability. The ones with nonvolatility which are capable of offering high-speed random accessibility also include ferro-electric RAMs using ferroelectric films. In these prior art semiconductor memories, they must have, without fail, transistors for use as the constituent parts or components thereof.
In a cell array configuration which is deemed ideal for use with RAMs, the use of rows and columns of select signal lines is inevitable as far as the cell array is organized into the form of a row/column matrix. If no wiring lines other than these row/column select lines are formed, then the cell array becomes simpler in configuration; however, in the prior art semiconductor memories, the cell array has been configured with increased complexities as a result of addition of power supply lines and data lines other than the above-noted signal lines. Additionally, memory cells are such that when miniaturization further progresses, it is difficult to maintain the characteristics thereof.
From these viewpoints, cells which utilize the nature of composition matter per se as a data state are expected to become more important in advanced memory technologies of the next generation in near future. As a promising one adaptable for use in such technologies, there has been proposed a phase-change or ovonic memory which utilizes a phase transition between crystalline and amorphous states of a chalcogenide-based glass material. The memory of this type utilizes the fact that a resistance ratio of the amorphous state to the crystalline state of the chalcogenide is as large as 100:1 or more to store therein such different resistance value states as information.
The chalcogenide glass has already been used in rewritable optical disks or else. Here, a difference of the refractivity of chalcogenide due to a phase change is used. This phase change is reversible, and any change can be controlled by adequately designing the way of heating, wherein the heating technique is controllable by the amount of a current flowing in this material. A trial for memory cells utilizing the feature of this material has been reported (for example, see Jpn. J. Appl. Phys. Vol. 39 (2000) PP. 6157-6161 Part 1, NO. 11, November 2000 “Submicron Nonvolatile Memory Cell Based on Reversible Phase Transition in Chalcogenide Glasses” Kazuya Nakayama et al).